Tuning a carburetor, part 2
Since we already tuned the choke, idle mixture, power valve, accelerator pump and secondaries, there’s only one thing left, the jets. On some Holleys the secondary jets are replaced by a nonadjustable plate. On Rochester Quadrajets the secondary jets (actually rods going into permanent jets) can be changed from the outside in 30 seconds while the primary jets require some dismantling of the carb. The primary jets should not be too rich since the affect daily driving and you fuel economy. Secondary jets on the other hand are rarely in use, so you can go quite rich on those.
Primary jets
You have to tune the primary jets first. This is done by a combination of how the car feels and looking at the exhaust and reading the plugs. Normally we’re against going on “feeling”, we’d rather measure, that’s more accurate. But here we have to. Because if the primary jets are too small (fuel/air ratio too lean) the car surges. So try driving steadily at a speed above where the idle circuit is involved, above 55 mph, does the car go slightly up and down in speed? If yes it’s a sign of too lean primary jets. Too light spark plugs will indicate the same. If on the other hand your primary jets are too rich, you’ll have smoke out your exhaust and dark colored almost black spark plugs.
If your car has neither a surge, smoke or abnormal spark plugs, you’re OK, no need to change primary jets. This will be the situation in most cases when the carburetor is meant for the engine, meaning that it’s the right size and a performance carb for a performance engine.
On the other extreme, if you have the surge, smoke or abnormal spark plugs symptoms and you change primary jets to solve it, if you end up having to change the jets more than 4 sizes something is wrong, you must have another issue not functioning or totally out of tune.
Secondary jets
These jets only come into play when you accelerate hard or drive fast. It that case economy is not that important, performance is the only target. So you can go a little rich, and most carbs are already tuned rich on the secondaries. You’ll rarely find more power by going leaner, unless the spark plugs are very dark after a run down the track and people tease you about the amount of smoke coming out of the exhaust, try bigger jets by testing, see section Testing. Keep trying bigger jets until performance doesn’t increases, then go back to the smallest jets that produced the most power
Here the #74 jets would be best choice.
This completes the ignition/carb tuning. You now have a well functioning engine that delivers power in all situations and should be fun to drive. And uses the least amount of gasoline possible for a performance engine. Well done!